The studies outlined for this project are aimed at analyzing the changes in actomyosin-like contractile proteins which accompany the aging of non- muscle cell cultures. The goal of these studies is to determine whether or not these changes can help to account for the cessation of motile behavior and growth of normal cells which accompany senescence in vitro. Actin containing microfilaments and their associated proteins represent the major ultrastructural component of the cytoplasmic contractile machinery. Therefore their structure, organization, intracellular localization and function are emphasized throughout the proposed research. Attempts to correlate function with structure through the coordinated use of microscopical, physiological, biochemical and immunological techniques will be employed.